The major cell surface glycoprotein fibronectin is often decreased on tumor cells and is involved in cellular adhesion. We have investigated its structure and functions. Cellular fibronectin and the closely related protein plasma fibronectin were compared. Although each had the same complement of functional domains for binding to the plasma membrane, collagen, heparin, actin, and DNA, they had at least three sites of apparent polypeptide difference. According to competitive inhibition studies, the plasma membrane binding sites for fibronectin may involve gangliosides or related molecules. A modulation of fibronectin functional activity by physiological concentrations of calcium was also demonstrated. Collaborative experiments suggested a role for fibronectin in mesenchyme cell movement, demonstrated specificity for only one region of fibronectin in mediating chemotaxis, and characterized the insulin receptor and the process of down-regulation. Our objectives will be to determine the structure and organization of the multiple active sites of fibronectin, the mechanisms by which it regulates cell behavior and embryonic differentiation, and the structure and function of its gene.